While not a 1x1 related answer: until a point where your partner has to respond, or act off of. There is conditions where a character realistically can't do much of anything about an action because they may not be in the same physical space or powerless or both writers may even understand this is what would happen otherwise.

I mostly write for Nation RP and in an RP environment with more characters or more people involved. But what I often tend to do if interacting with other people talk about the situation before hand and work out a general ground work so all parties involved know what each of us will be doing. For example I'm not going to "lead" someone to a hallway and then stop and let them write about being lead down the hallway or consenting to being lead down the hallway before being walked down the hallway is written. If both of us know that being at the end of the hallway is where the next major part of the scene is going to be; it'll be there. In this case there's also a notion of one of us being the "host" of that scene or post, where it's primarily my own but the other person is there to write the bits that must be dictated and acted on by the other player. To go off of the hallway bit: I might lead someone down the hallway but if at the end they will be commanded to kill someone then that is when the other character would be allowed their autonomy to refuse or accept the command, give their thought process and actions on how they do it or even arguments.

In the longest of terms both of us may know generally what we want at the very end of all of this.